Obtaining information on cardiovascular parameters is of great importance for ascertaining people's state of health. The availability of an apparatus which can obtain said information beat by beat continuously, simply and comfortably, without requiring the user themselves or an assistant to have any specific abilities or training, is of great interest. Particularly, when the measurements are not taken in clinical or health care environments it is very desirable for the subject not to need any aid to be able to take the measurement.
Among the noninvasive measures providing information on the cardiovascular system, the bioelectric signal measurements are some of the easiest to obtain, above all if taken with dry electrodes (without conducting gel). This practice limits the measuring areas to the limbs (arms and legs) because the mechanical contact with the electrodes may be made by fixing them or holding them in the hand or resting the hands or feet on them. The application of electrodes to other parts of the body, the thorax for example, requires them to be held with means achieving a sufficient pressure to guarantee a good contact, which involves an obvious inconvenience and requires time for placing them.
Two bioelectric signals that provide information on the cardiovascular system are the electrocardiogram (or ECG) and the electrical impedance signals measured in volumes of the body where there is a variation attributable to the blood flow. The measurement of volume changes (plethysmography) based on measuring the electrical impedance is called impedance plethysmography (IPG). Plethysmography based on measuring the light absorbency is called photoplethysmography (PPG).
The ECG and IPG provide information on the cardiovascular system not only separately but also jointly. To be precise, the time taken by the arterial pulse wave (involving a simultaneous change of volume) to reach a part of the body depends not only on the distance between said part and the heart, but it also depends on the diameter, thickness and stiffness of the arteries, and on the rheological properties of the blood. A time including said information is the one known as PAT (pulse arrival time), which is of great diagnostic interest. See, for example, Eliakim et al, Pulse wave velocity in healthy subjects and in patients with various disease states, American Heart Journal, vol. 82, no 4, pp 448-457, October 1971. Particularly, the PAT measured between the R wave of the ECG and the foot (starting point of the rapid rise associated with the ventricular systole) of the photoplethysmography (PPG) in a finger of one hand is frequently used to estimate the systolic pressure, for example, as described by Chen et al., in “Continuous estimation of systolic blood pressure using the pulse arrival time and intermittent calibration”; Medical and Biological Engineering and Computing, vol. 38, pp. 569-574, 2000. Since both the PPG and the IPG measure changes of volume, the wave forms of both signals are analogous, and therefore the IPG has been used as an alternative signal to the PPG to measure time intervals related with the propagation of the pulse wave in the arteries.
In the document by Bang et al. “A pulse transit time measurement method based on electrocardiography and bioimpedance” Biomedical Circuits and Systems Conference (BioCAS) 2009, pp. 153-156 the time elapsed between the ECG R wave, obtained with an electrode on each arm, and the IPG peak, obtained with four electrodes on the forearm, was measured and this time was compared with the time elapsed between the ECG R wave and the PPG peak. The correlation between both times was excellent. Although this method proposed by Bang et al. has the advantage of not requiring electrodes on the thorax, the use of conventional electrodes (with conducting gel) adhered to the arm to obtain the ECG and IPG makes the process slow and uncomfortable.
Another document where the measuring of physiological parameters by the ECG and IPG without the necessity of locating electrodes on the thorax is described in U.S. Pat. No. 6,228,033 for Apparatus and methods for a noninvasive measurement of physiological parameters, to Kööbi et al., 2001. In this patent the IPG is preferably obtained by injecting a current between both arms and both legs at the same time, and detecting also at the same time between both arms and both legs. In this regard, see FIG. 1 herein, where the injection electrodes are the pair 31 and the pair 32, and the detection electrodes are the pair 11 and the pair 12. In a preferred embodiment a detection electrode 11 is about 5 cm from an adjacent injection electrode 31. With these electrode connections, it is stated in said patent that the IPG obtained reflects above all the overall impedance changes between arms and legs, which will be proportional to the pumping out of blood from the left ventricle. To obtain a distal pulse wave, Kööbi et al. obtain the IPG in a segment of one limb, by using a further two electrodes (21 and 22). They obtain the ECG with the same electrodes (pair 11 and pair 12) with which there is detected the potential difference created by the injected current to measure the impedance without needing thoracic electrodes. The same document describes that the injection of current to obtain the overall IPG is always at least between one arm and one leg; a possible arrangement of the electrodes according to this embodiment is shown in FIG. 2 herein where the injection is through the electrode 31 and electrode 32 and the detection is through the electrode 11 and the electrode 12. However, according to Kööbi et al, even in this embodiment wherein injection occurs only through one arm and one foot, to obtain the distal pulse wave by the IPG in a segment of one limb a further two detection electrodes (21 and 22) disposed along said segment are still necessary. It is concluded, therefore, that according to the method described in U.S. Pat. No. 6,228,033 simultaneously to obtain a distal pulse wave and the ECG, at least six electrodes are required, although there is obtained also another IPG basically reflecting the impedance changes in the thorax. To obtain the transit time of the pulse wave to a distal segment, they calculate the distance between the peaks of both impedance signals obtained, one from the voltage detected between the electrode 11 and the electrode 12 (FIG. 2) or between the pair of electrodes 11 and the pair of electrodes 12 (FIG. 1) and the other from the voltage detected between the electrodes 21 and 22.
On the other hand, this Kööbi et al. patent is contemplated for clinical environments and perhaps for this reason they consider the possibility of using electrodes with gel as an advantage, since they are common in electrocardiography. In fact, the four electrodes at least necessary for the limbs (11, 12, 31 and 32 in FIG. 2) could be replaced by dry electrodes. On the other hand, if the two electrodes (21 and 22 in FIG. 2) required for obtaining a local pulse wave by the IPG in a segment of a limb were dry, they would have to be held in place by a strap or other similar means. Furthermore the need always to have a connection with at least one arm and one leg does not favor the design of a system so compact as may be needing only both hands or both feet.
The use of electrodes on the limbs and on different parts of the thorax to measure therebetween the overall electrical impedance and the electrical impedance at different sections of the body, and the changes thereof over time is also described in the document WO 2005/010640 “Non-invasive multi-channel monitoring of hemodynamic parameters” to Tsoglin and Margolin, 2005. But, to measure the peripheral blood flow they use, for example, additional electrodes on one finger (pag. 13 and FIGS. 1, 2I, 3A, 3B, 3C, 4C and 5). Furthermore, although some of the electrodes used for measuring the bioimpedance are also used to obtain the ECG, they do not do so simultaneously, but the apparatus includes a switching circuit (member 29 in FIG. 6 of the document) connecting the electrodes for carrying out one function or the other, but never both together, whereby it is not possible to obtain cardiovascular information from the combination of the simultaneous measurements of both.